I understood that medical degrees were pass/fail, rather than being classified in the way other degrees are. So she might have known that her exam results were top 10% of her cohort, but she won’t have graduated with any ‘better’ a degree and that won’t be known to anyone else?
This is correct: in general, most medicine degrees are honours degrees without classifications, but typically the top 10% or so will graduate with distinction. The exception will be those that include other degrees within the programme, i.e. Oxford, Cambridge, St Andrews, Nottingham and any with compulsory intercalation (Edinburgh, King's, Imperial, UCL), where the additional degree will be classified.
However, the way the allocation of foundation training posts worked before this year was that each student had an educational performance measure (EPM) based on their ranking within their cohort at their medical school (expressed as a decile for exam results up to the penultimate year of the programme) plus any additional credit-bearing markers such as publications, additional degrees, etc. This was combined with a score for the situational judgement test to give an overall ranking score. The highest-ranked applicants had a greater probability of being allocated their more preferred foundation schools.
This was removed this year, and the allocation was based first on applicant's preference and then on a random allocation. (Although this seems not to have worked properly, as some applicants did not get either of their first two preferences even though they were undersubscribed.)
There are strong arguments for changing from the previous system, not least that students who had the greatest need for support after graduating were the ones most likely to end up in the foundation schools least able to provide it. It's difficult to define the "best" medical graduates, as these aren't necessarily the ones with the best exam results (because exams are an inherently flawed form of assessment), but it is important to make sure the best doctors don't all end up working in a small number of prestigious centres, for reasons @ErrolTheDragon has spelled out. However, the expectation that newly-qualified doctors will just go where they're sent and do what they're told is likely to accelerate the rate at which they emigrate. It isn't a privilege to be a doctor, nor is it a "vocation" (I hate that word): it's a means of making a living that has been worked hard for and the monopoly employer should show some recognition for doctors' need to have some idea of where & how they can plan their future.